Ran Mōri and her friend Sonoko Suzuki are invited by Wakamatsu, one of the two guys they had met the other day, to join him, and a couple of friends a valentines day party, to which they agreed to go to. They arrive as well as Conan, who sneeked on to the premises as well as getting caught there. Tragedy soon strikes the party as Katsuhiko lies on the ground dead from poison. Naomichi had originally confessed to killing Katsuhiko, but Conan figures out he is not the true murder, but is instead Katsuhikio's own mother. The mother eventually admits to the crime, needing the life insurance money on Katsuhiko's name, and is put away.

When a writer kills his associate during a festival, Conan has to uncover the hole in his alibi and prove him guilty.